The present invention relates to a line thermal printer operative to supply electric energy to a plurality of linearly arranged resistive elements to effect conversion into thermal energy to carry out direct printing or transfer printing on a recording medium, such as a sheet of paper medium.
There has been known a line printer utilizing a thermal head operable according to an image signal for effecting color printing on a paper sheet medium. As shown in FIG. 2A, a platen A is provided for feeding a printing sheet P, and a thermal head C is opposed to the platen A along its rotational axis to effect printing. As shown in FIG. 2B, the thermal head C is comprised of a plurality of linearly arranged resistive elements B.sub.o -B.sub.n. FIG. 2C shows a driving circuit of the resistive elements B.sub.o -B.sub.n.
In the conventional line printer, the respective resistive elements B.sub.o -B.sub.n of the thermal head C are driven to generate thermal energy according to gradation level data of an image signal fed from a host computer, line-sequentially so as to develop color tones in dot patterns on the printing sheet P. In such construction of the line printer, the plurality of resistive elements B.sub.o -B.sub.n must be controlled to generate the same impression density for the same inputted gradation level data so as to avoid dot density variation on the printing sheet P.
However, individual resistive elements B.sub.o -B.sub.n of the thermal head C have different resistance values due to patterning errors in the production process, in an allowable range of .+-.20%, thereby resulting in actual print density variation among dots of the resistive elements B.sub.o -B.sub.n.
Further, as shown in FIG. 2C, the thermal head circuit contains a common resistor component r between adjacent resistive elements B.sub.o -B.sub.n so that each of the resistive elements B.sub.o -B.sub.n receives varying driving voltage due to the common resistor components r even if the resistance value differences of the elements B.sub.o -B.sub.n are compensated or eliminated.
Moreover, as shown in FIG. 3, the thermal head circuit is constructed such that the common voltage V.sub.o is applied through the end elements B.sub.o and B.sub.n so that the effective driving voltage gradually decreases from the end elements to the central element to thereby cause variation of the print dot density.
Furthermore, the thermal head C may not contact uniformly with the print sheet P due to surface unevenness of various components, thereby causing print dot density variation even if the driving current is supplied to the elements for the same duration, thereby disadvantageously degrading printing quality.
In order to eliminate the print dot density variation due to the above noted various reasons, conventionally the resistance values of the respective resistive elements B.sub.o -B.sub.n are provisionally measured and external components are connected to compensate variation of the resistance values. However, such construction for coupling a plurality of external adjustment members is very complicating thereby raising production cost.